Former education secretary Nicky Morgan has broken ranks to warn the that government’s plans for a new wave of grammar schools risks undermining the Conservatives’ education reforms.

Ms Morgan, who was sacked from her role at the Department for Education by Theresa May in July, has become the first high-profile Tory MP to speak out against the plans.

The prime minister this morning set out a raft of hugely controversial plans that would dramaticallyexpand the number of grammar schools in England.

The proposals have attracted fierce criticism from teaching unions, heads’ leaders and universities but Ms Morgan’s intervention gives a glimpse of the difficulty Ms May will face in pushing through her reforms.

Writing on her Facebook page, Ms Morgan urges the government to abandon its plans for greater selection in the system.

“The prime minister is absolutely right to place creating a more meritocratic society at the heart of her agenda for government… However, I believe that an increase in pupil segregation on the basis of academic selection would be at best a distraction from crucial reforms to raise standards and narrow the attainment gap and at worse risk actively undermining six years of progressive education reform,” the former Cabinet member writes.

“The evidence is now incontrovertibly clear that a rigorous academic education does not need to be the preserve of the few.”

Ms Morgan highlighted work being done by comprehensive schools such as Harris Academy in Peckham, ARK Charter Academy in Portsmouth and Dixons [City Academy] in Bradford.

“Instead of pursuing greater selection, I would urge the government to build on the reforms of the last Government and to reaffirm the focus outlined in the Educational Excellence Everywhere White Paper on tackling areas of entrenched academic underperformance,” she adds.